The success of the permanent anchorage of hip joint components is linked to the publications by Sir John Charnley, who cemented the components into place using bone cement, such as the publication that appeared in the Journal of Bone and Joint Surgery: Charnley. J., (1960), Anchorage of the Femoral Head Prosthesis to the Shaft of the Femur JBJSB 42: 28-30. The combination of the cemented plastic socket also goes back to J. Charnley (Low Friction Arthroplasty of the Hip: Theory and Practice, Heidelberg, N.Y., Berlin: Springer. However, combination with plastic resulted in unexpectedly high wear.
Therefore, researchers attempted to find combinations for the articulation that could be expected to have low wear. Industrial experience told them that the smoothest surfaces could be achieved with ceramics. The articulation of ceramic against ceramic currently allows one to expect wear that will be 200 times less than that with a polyethylene-metal head. However, such an articulation has a problem: Ceramic cannot be subjected to flexural or shear loads.
The combination of ceramic and ceramic led to the development of metal implants that protected the ceramic from flexural and shear loads and that could be anchored by press-fitting them into bones. This surface press-fit, which can also be ground with diamond-faced tools (PA D 199 60 707.9) does lower the center of rotation (CR) of the socket into the lower area of the acetabulum, and this leads to a medialization of the legxe2x80x94a displacement inwards. In order to prevent this, researchers have been trying the reconstruct the CR. This was successful in the present invention.
There are various ways to anchor the socket. One of the most commonly used is to cement in an UHMWPE (Ultra-High-Molecular-Weight PolyEthylene) socket with bone cement, as described above. It was assumed that cementing the socket would be governed by the same laws and therefore have the same success as cementing the femur component. This was not the case. The socket is cemented under very different conditions. The plastic socket is cemented onto a relatively dense bone, which means that the bone cement cannot penetrate at all into the bone if no anchorage holes have been made. Such anchorage holes, however, represent hourglass connections that do not provide sufficient anchorage since the spongiosa structure in the acetabulum initially has a closely spaced honeycomb, which becomes more widely spaced as one moves in the proximal direction toward the spinal column/pelvis connection. Filling occurs xe2x80x9cagainstxe2x80x9d the morphological structure, and as a result both the rigidification of the bone structure as well as the anchoragexe2x80x94the so-called interlockingxe2x80x94is inadequate.
Successful attempts have been made to grind open the dense top of the socket without destroying the stability of the lightweight design (sandwich construction). However, a diamond instrument must be used for this purpose, and the bone cement must be drawn in. Since this technology has only become available recently (DPA 199 60 707.9), and since one also has to deal with the wear problem encountered in plastic sockets, researchers have recently been trying to test other potential anchorage systems.
One of the most commonly used of these systems is the so-called screw principle, in which the artificial socket is cut directly into the bone without an attachment means and is screwed down. However, the screw threads result in bone necrosis and pelvis fractures.
Attempts were also made to enlarge the anchoring surface in the bone and to allow the bone to grow in (porous ingrowth principle), but here too the loosening rate was high, and the implant became encapsulated in the interconnective tissue. Combining this approach with a threaded connection also failed to solve the problem.
Researchers first had to study the morphology and the function of the lunate fasciaxe2x80x94the crescent-moon-shaped articular surface of the socket. This led to a physiological anchorage principle: the principle of providing pretension in the bone. This was not new, but it was easy to implement in the case of the socket since the deformation of the joint surface is nearly concentric.
It was therefore possible to press-fit a nearly spherical component in the acetabulum, the name used for the socket when it is located in the pelvis. The details of such a socket are described in DPA 100 03 543.4.
The surface press-fit necessitates that the socket be blocked over a large area behind the equator of a virtual socket ball; when this is done, the center of rotation is always moved downward into the socket. This was not given due attention, but it can also be an advantage for the bony anchorage, since the lever arms for a tipping movement are shorterxe2x80x94in other words, the so-called socket offset is shorter. But in any event it means that the leg as a whole is medialized further, which can result in pelvitrochanterian insufficiency. This means that the muscles become relatively long and have an unfavorable lever arm. For this reason, attempts were made to find solutions that combine fixed seating with a reconstruction of the position of the center of rotation. EP 0 694 294  B1 describes an insert which, upon first examination, appears to be able to accomplish this goal, but in fact it cannot. The object of that invention is to loosen the joint socket from the exterior surface cone, where a margin is used as an opposing bearing, which allows the outward bracing. An insert of this type cannot be used for the invention described below since abutment on the perimeter of the shell prevents a press-fit seat. However, the following invention was able to solve this problem simply and elegantly.
The invention comprises a modular implant that has an outer shell as shown in FIG. 01/10. This outer shell is flattened at the topdome (FIG. 01/11), and has a spherical band that is the anchoring zone (FIG. 01/12). The inside contour corresponds to a precise cone having a defined angle (FIG. 01/13). The surface of the titanium socket that is represented in this example has a roughness of 80-100 microns (FIG. 02/16), and the surface of the cone has a defined undulation for holding the ceramic (FIG. 01/20) or the insert module (FIG. 01/30) and (FIG. 01/40).
The shell (FIG. 01/10), which in this case is made of titanium, has perforations in its topdome to reduce the mass and to increase the overall elasticity of the design (FIG. 01/14). The holes in the wall of the titanium implant (FIG. 01/15) have cushioning properties relative to the bone, as can be shown by finite element calculations.
The module, which can be inserted in the titanium shell (FIG. 01/10) and which is made of the same material (FIG. 01/30), has an annular support (FIG. 01/31) that is designed in such a way (FIG. 01/32) that a concentration of stress does not occur at the point where the spherical shell is attached. The base (FIG. 01/33) of the module is open; the cone (FIG. 01/34) has a surface that is specifically undulated to hold the ceramic insert and is undercut (FIG. 01/35), to achieve subsidence, a term that refers to the self-setting ability of the implant. In order to achieve long-term stability, the module and the articulating insert can be rigidly connected to each other (FIG. 01/80).
The module (FIG. 01/30) thereby experiences a relative change in the offset of 0 mm relative to the pure ceramic insert (FIG. 01/20) of xe2x88x921 mm; and the module (FIG. 01/60) may have an offset of +4 mm. The offset in a socket refers to the distance from the median plane to the center of rotation, and it is inherent in this definition that the topography of the physiological center rotation is defined as a 0 mm. A module having an offset of 7 mm has also been designed, as shown in FIG. 01/40. The module may be constructed of a single piece and it may be rigidly attached with components made of ceramic, HDPE, metal, or some other abrasion-resistant material, which would be advantageous.
In the case of the titanium/HDPE versions, this change in the offset is easier to incorporate in the design of the HDPE insert, as is shown in FIGS. 01/50, 01/60, and 01/70. The conical exterior of the insert (FIG. 01/61) has a specific undulation for the mounting area in the titanium insert. This undulation holds the plastic insert in an interlocking manner, FIG. 01/30 and FIG. 01/40. The interior surfaces of the plastic insert have a specific smoothness; in the case of the one-part design, they are rigidly connected to the metal.
FIG. 4 illustrates a setting instrument for the atraumatic-press fit anchoring of the socket in the patient""s pelvis. The instrument 200 includes a handle assembly 220 having a handle 222 at one end provided with a mallet impact element 223 and an end piece 221 at its opposite end. The end piece 221 has a conical end that can fit in the conical entry of the outer shell 10. A central rod 210 projects from the handle assembly 220 beyond the end piece 221 and is provided at its free end 211 with means for attaching the shell 10 to the setting instrument. The central rod 210 is moveable in the impact direction relative to the handle assembly with a spring like action. The central rod to project beyond the end piece 221.